Closed tubes with connected attachment fittings are often used in vehicle suspension dampers. A damper operates in vehicle suspensions as a damping device controlling the sprung (body) and unsprung (wheels) masses of a vehicle by reducing loads and vertical accelerations that are normally transmitted from the wheels to the body. Damping is accomplished by converting kinetic energy into thermal energy and dissipating the heat. Conventionally hydraulic dampers include a piston with a connected piston rod slidably carried in a fluid-filled tube and separating the tube into extension and compression chambers. A rod guide at the top end of the tube closes the extension chamber and slidably engages the piston rod. The piston rod and the tube are provided with attachment fittings for connection to the sprung and unsprung masses. Due to the damper's operating environment, the fittings must be securely connected to withstand the applied forces and a fluid-tight closure of the tube must be established and maintained during operation.
The various types of known vehicle suspension dampers use several different methods to close the ends of the tubular sections. In applications such as MacPherson struts, a cap (base cup) is placed in the end of the tube, and the tube and cap are connected together and sealed by gas metal arc welding or resistance seam welding to complete the assembly. These welds produce a strong joint and a leak-free seal of the unit. Other methods of closing damper tubes utilize a series of extrusions with intermediate annealing operations to produce a closed-tubular component from a solid steel blank. Closing a tube end over an arbor by means of an external forming tool is also known. These methods, which often undesirably require the introduction of heat from an external source require extensive cleaning operations to remove oxide coatings and other contaminants. In some cases heat is required to permit adequate formation of the steel tubing material while avoiding failure in tension due to extraordinarily severe metal working conditions. Accordingly, a new method of producing a closed tube and attachment fitting assembly would preferably operate without the introduction of external heat during forming operations and would result in a securely sealed joint.